A three dimensional integrated circuit (3D IC) is an integrated circuit (IC) built by stacking wafers and/or dies and interconnecting them vertically so that they behave as one IC. 3D ICs can pack a deal of functionality into a small “footprint.” The different devices in the stack may be heterogeneous, e.g. combining complementary metal-oxide-semiconductor (CMOS) logic and dynamic random access memory (DRAM) into one IC. In addition, critical electrical paths through the IC can be shortened, leading to faster operation.